This invention relates generally to an optical finger image generating apparatus and, more importantly, to an optical finger image apparatus which generates an image from a scan of a finger object without a platen.
Fingerprint apparatus capable of generating a fingerprint image which can be encoded into machine readable signals are known in the art. Examples, of such apparatus are described in U.S. Pat. No. 4,322,163. This apparatus generates a fingerprint image using substantially collimated light as an interrogating light beam which is displaced across a platen upon which a finger is supported. The finger on the platen modulates the interrogating light beam to provide a reflected light beam which has fingerprint information. Optical scanning means are used which cause the interrogating light beam to scan the fingerprint object carried by the finger on the platen. The modulated light beam is imaged onto an array of photo-electric transducers to produce a series of output signals indicative of the modulated information. The output of the array is serially interrogated at successive scan positions to provide a set of signals containing fingerprint information.
In the art of optical finger image processing, it is desirable to keep the system as inexpensive as possible and the size of the system as small as possible while ensuring that the reliability of the system is maintained. In order to get a useful fingerprint image the background signal must be kept as low and as steady as possible. Further, there must be a high resolution image of the fingerprint object. Prior art apparatus while providing a reliable system with high resolution and a controlled background signal uses platens upon which the finger being scanned is supported. Although the platens help to control the background signal and aid in resolution they also create problems. With use, deposits of oil, grease and dirt build up on the platen which affect the fingerprint image. Further, the pressure of a finger against the platen distorts the fingerprint object and the distortion is somewhat different each time the finger is applied to the platen.
These distortions arise out of at least four types of situations. There is a tendency for the ridges to close down on the valleys in the higher frequency portions of a finger when the finger is pressed against a platen. The positioning of a finger on the platen requires movement of the finger along the platen to a pre-determined position and this generates a slip-stick distortion due to the friction of the finger on the platen surface. A third type of distortion is latent image distortion which arises from the latent images left by previous applications of a finger to a platen. A fourth type of distortion occurs because of the interaction of the finger oil with the platen. This last distortion is particularly severe with fingers which generate a great deal of oil. All these distortion situations tend to be non-repeatable so that the finger image varies as between applications of the same finger to a platen. This complicates analysis and requires relatively expensive processing to compensate for these distortions.
U.S. Pat. No. 3,614,747 to Charles Sadowsky discloses a fingerprint apparatus which uses regular light to scan a portion of a finger through a slit. The Sadowsky apparatus uses and must use a point illumination to keep the background light level (that is, the base line in the electrical signal) as low as possible. Sadowsky employs a shadowing technique to generate differential illumination of ridges and valleys by scanning an interrogating point of light along a single slit aperture.
It is an object of the present invention to provide an optical finger image processing apparatus which produces accurate fingerprint images without using a platen.
Another object of the present invention is to provide such an apparatus which avoids swamping out the image signal with the base line signal.
It is a further object to provide such an apparatus which is relatively inexpensive, reliable and provides a repeatable image.
Still a further object is to provide such an apparatus which is relatively small in size.
It is an object of this invention to meet the various objects stated above in a device that will be effective with the wide range of finger surfaces that exist in the population including fingers in which the valleys are shallow and fingers where the valleys are deep and including fingers where the ridges are quite narrow and fingers where the ridges are relatively broad.
Brief Description of the Drawings
FIG. 1 is an optical and mechanical schematic of the device of this invention showing a finger in position being interrogated by a light beam 16.
FIG. 2 is a partial illustration of the FIG. 1 apparatus looking into the tip of the finger. FIG. 2 illustrates, in exaggerated form, the offset d between the illumination plane and the axis of rotation of the optical interrogating and detecting unit.
FIG. 3 is a schematic idealized representation of the typical relationship between the ridge zones R and valley zones V of an undistorted finger.
Brief Description
In brief, the device of this invention provides for the direct reading of the surface of a finger with a coherent collimated interrogating light beam. The light beam reflected from the finger surface that is interrogated by the coherent light beam will contain fingerprint information. This reflected light beam is incident on a linear array of photo-electric transducers. Thus, at any one moment in time, only a thin line across the finger is imaged at the linear array of transducers. Accordingly, only a thin slit of the interrogating light beam is required to produce imagery at the transducer array. Thus, the interrogating light beam is shaped by cylindrical beam forming lenses into a flat planar like beam that impinges on the finger as a slit of light. The plane of the interrogating light beam and the plane of the reflected modulated light beam are co-planar. The line of interrogation is roughly parallel to the axis of the finger being interrogated.
The co-planar interrogating light beam and reflected modulated light beam form an illumination plane. This illumination plane is scanned around the finger in a circumferential direction perpendicular to the illumination plane. The optical scan involves rotation of the illumination plane so as to maintain the illumination plane parallel to a pre-determined axis, which axis is the nominal axis of an idealized cylindrical finger.
The rotation of the illumination plane around the surface of the finger provides an output image which is the equivalent to a full roll of a fingerprint in a fingerprint inking technique. Accordingly, the arc of rotation of the optical interrogating and detection unit is sufficient to provide this full roll finger image.
It is because of this rotation of the illumination plane around the surface of a finger not supported on a platen that requires the co-planarity between the interrogating light beam and the reflected modulated light beam. Since the finger surface is not a uniform cylinder, there would be substantial geometric distortion of the image at the array during the course of the scan. In order to make sure that the longitudinal line along the finger that is being interrogated appears at the linear array of transducers, it is necessary that the interrogating plane and the reflected modulated plane be the same.
A laser diode provides a coherent light beam which is shaped into a substantially collimated interrogating plane of light by an assemblage of spherical and cylindrical lenses. The interrogating light beam is scanned across the finger to interrogate that portion of the finger which extends beyond the support and to produce the reflected light beam having fingerprint information. Within the illumination plane, the collimated interrogating light beam is at an angle to a line normal to the surface of the finger being interrogated. Thus, within the illumination plane the interrogating light rays strike the surface of the finger at a slight off normal angle. In addition, the illumination plane is offset slightly from the axis of rotation so as to assure that the illumination plane is not quite perpendicular to the surface of the finger at the line of interrogation. The combination of the off-normal angle within the illumination plane and the offset of the illumination plane from the nominal axis of the finger assures that there will be shadowing in two dimensions.
The optical system, including the laser diode, the interrogating beam, the beam forming lenses, the transducer array and the focusing lens assembly which focuses an image on the transducer array are all mounted on a platform which rotates about the nominal axis of the finger so as provide a scan which can run from nail to nail.
The effect of the offset between illumination plane and finger axis is to provide a shadowing result along a circumferential line around the finger. This is similar to the shadowing on an axial or longitudinal line along the finger created by the off-normal incidence of the light within the illumination plane.
What has been observed in testing this invention to minimize and maximize shadowing is that there are two phenomenon which reinforce one another to provide a significant image for a wide range of finger surfaces.
The two phenomenon which the device of this invention appears to optimally employ so as to provide meaningful imagery across the wide range of fingers that exist in the population are the phenomenon of shadowing discussed above and the phenomenon of constructive and destructive interference. Although the exact explanation for the manner in which this invention provides observable significantly improved results is not fully understood, it is believed that there is a significant degree of constructive and destructive interference at the image plane by the reflected coherent light such as to substantially enhance the imagery provided by shadowing.
In particular, the coherent light reflected from the ridges constructively interferes to provide bright imagery at the array and the coherent light reflected from the valleys destructively interferes to provide dark imagery at the array of transducers. Notes on Terminology
Illumination Plane
The illumination plane as used herein refers to a narrow planar like zone which encompasses the linear array of photo-electric transducers. Thus, the plane has thickness and that thickness is substantially determined by the aperture of the diodes that constitute the array. The interrogating light beam is preferably shaped to extend only slightly beyond that plane. By so shaping the interrogating light beam, the light is most efficiently used. That is, the least light is wasted and the least power required As a consequence of this shaping of the interrogating light beam, the reflected modulated light beam bearing fingerprint information also does not extend substantially beyond the thickness of the illumination plane. All light outside the thickness of the illumination plane will be outside of the aperture of the transducers and not contribute to the image formed.
If the interrogating light beam were not shaped to closely conform to this illumination plane, the invention would still operate and there would be a diminution of the intensity of the illumination and thus a reduction of contrast between the light and dark pixels produced by the transducer array. But there would still be an illumination plane; which plane would continue to be defined by the location and aperture width of the transducer array and the optical axis of the imaging lens.